We do not use Captcha for our registration. Aside from the unnecessary costs, it's (as y'all seem to agree) a pain in the butt for users. Our registration process involves a bank of questions that should be trivial for RPGers to answer. I won't get overly detailed, but one example of a question we might use would be "What is the traditional 3 letter abbreviation for the intelligence stat?"

Myrthos changes the questions in the bank from time to time as it appears the spambots eventually "learn" our questions. When Corwin and I start seeing a rash of spambots, we trek to the top of Mount Myrthos, toss a goat into the lava, and humbly request that new questions be set up. Then we start dodging the lightning.